The present study was done to prove our hypothesis that long-lived acquired resistance of humans to Mycobacterium tuberculosis is ascribed to the transformation of M.tuberculosis into L-form M.tuberculosis in macrophages localized at granulomatous lesions. To determine whether mycobacteria can transform into L forms in vivo, murine BCG infection model was used.Results obrtained herein were as follows :1) BCG transformed into L forms in Bcg^r mice after 60 days of infection, and treatment of infected Bcg^s miced with isoniazid (INH) induced the transformation of BCG into L forms.2) L-form BCG was detected in mononuclear cells of granulomatous lesions in the lung by the in-situ PCR.3) IFN-gamma was responsible for the transformation of intracellular BCG into L forms.4) Bcg^s mice immunized with L-form BCG or colonized with L-form BCG were immune from an intravenous infection with a lethal dose of BCG.5) Reversion of L-form BCG to parental BCG was achieved by treating L form-colonized mice with estrogen or TNF-alpha. This reversion was associated with the enhanced production of TNF-alpha and TGF-beta.6) In L form-colonized mice, mRNAs for IL-1, IL-12 and IFN-gamma were strongly expressed after an intravenous infection with a lethal dose of BCG.These data strongly suggest that transformation of BCG into L forms contribute to the maintenance of acquired resistance to infection with a lethal dose of BCG.Such a mechanism is possibly applied to human mycobacterial immunity which can last over a long period.